So-called “unbonded” flexible tubular pipes are described in the normative documents API 17J, “Specification for Unbonded Flexible Pipe”, API RP 17B, “Recommended Practice for Flexible Pipe”, API 17J, “Specification for Unbonded Flexible Pipe”, API 16C, “Choke and Kill Systems”, and API 7K, “Rotary Drilling Hose”, published by the American Petroleum Institute. Those pipes comprise several superposed layers of metallic materials and plastic materials, giving them their mechanical properties and their sealing properties with respect to the hydrocarbon being transported and with respect to the environment.
They also generally comprise, from the inside to the outside, a metallic carcass made of an interlocked spiral metal strip, a pressure sheath of polymer material, a short-pitch helical winding of a metallic wire forming a pressure vault, at least one web of tensile armor layers realized by a long-pitch winding of a plurality of metallic wires around said pressure vault, and an external protection sheath.
When the pipes are subjected to strong hydrostatic pressures, and especially in the deep sea, once the internal pressure of the pipe decreases too much, just below the hydrostatic pressure, they are subjected to the “inverse bottom effect”. The pipe then undergoes axial compression forces tending to shorten it. When, in a dynamic and turbulent environment, transverse stresses on the pipe are added to this, the armor wires of the tensile armor webs then tend to buckle laterally and eventually disorganize the pipe locally in an irreversible manner. These armor wires then form so-called “bird cage” structures locally.
To remedy this, one winds a holding strip in a short-pitch helix around the outermost web of tensile armor wires. These holding or reinforcing strips are made of organic fibers, such as aramide, and they will then restrain the effects of radial buckling and swelling of the wires of the tensile armor webs. Moreover, it has been proposed to coat the holding strip with a polymer material in order to lengthen its lifetime. The polymer material is then integrated with the holding strip and forms a single whole with it. One may refer to document WO2008/135 663 which describes such a holding strip. However, the strip so coated is not protected against degradation in all operating conditions, and especially very deep marine conditions.
It has also been proposed to create steel wires to define the holding strips. However, this makes the pipes significantly heavier.